Unilamellar metallic nanosheets possess superiority for electrochemical energy storage and conversion applications compared to the few-layered bulk and semiconducting counterparts. Here, we report the utilization of unilamellar metallic 1T phase MoS 2 nanosheets for efficient sodium storage and hydrogen evolution through a MoS 2 /graphene superlattice. The superlattice-like assembly composed of alternately restacked unilamellar MoS 2 and modified reduced graphene oxide nanosheets was prepared by a facile solution-phase direct restacking method. As an anode for sodium storage, the MoS 2 /graphene superlattice anode exhibited an excellent rate capability of ∼240 mA h g −1 at 51.2 A g −1 and a stable reversible capacity of ∼380 mA h g −1 after 1000 cycles at 10 A g −1 . In addition, a low onset potential of ∼88 mV and a small Tafel slope of 48.7 mV decade −1 were attained for the hydrogen evolution reaction. Our findings are important for further developing the potential of 2D nanosheets for energy storage and conversion.

We reveal that chemically exfoliated MoS nanosheets undergo lateral fracture and aggregation upon prolonged storage of the dispersion in ambient air, which was found to be associated with the reoxidation of the nanosheets. Such nanosheet degradation could be effectively prevented by storing the sample in an inert atmosphere to suppress the reoxidation process.

Chemical exfoliation of molybdenum disulfide (2H-MoS) for preparing high-yield single-layer sheets has attracted considerable attention in recent years. However, the stability and nature of the resulting nanosheets are poorly understood. Storing the dispersion in ambient air brings about the reoxidation of the nanosheets, releasing their residual negative charges into the environment. The reoxidation facilitates lateral fractures and destabilizes the dispersion. In-plane X-ray diffraction of the nanosheets indicates that they have a 1T structure with a 2D √3 × 1 rectangular cell as the intrinsic structure for chemically exfoliated MoS. We found that the 1T structure was preserved after reoxidation upon aging the dispersions in air, suggesting the formation of metastable neutral MoS. The changes in the chemical nature of the nanosheets can be monitored by X-ray diffraction of the restacked nanosheets. The restacked nanosheets, obtained by drying the freshly prepared dispersion, exhibited an expanded bilayer hydrate structure, accommodating Li ions. On the other hand, dried samples from the aged dispersions were substantially composed of a deintercalated phase and the bilayer hydrate. Upon prolonged aging, the former phase became predominant with total disappearance of the latter. This evolution suggests that the reoxidation occurred sheet by sheet with a direct restoration of the original oxidation states of the nanosheets, whereas the oxidation states of the nanosheets can be discrete at 4+ and (4 - δ)+.